The VP2 protein exhibits cross-interaction to the VP1 protein in norovirus GII.17

Predominance of Recombinant Norovirus Strains in Greece, 2016-2018

GII.4 noroviruses have caused the overwhelming majority of norovirus-related gastroenteritis cases during the past two decades. However, a trend towards the emergence of new genotypes and novel GII.4 variants provided the impetus to explore further the changing patterns in norovirus epidemiology during the present study. Genotyping of 60 norovirus strains detected during a period of 33 months (January 2016-October 2018) was performed on the basis of the capsid VP1-coding ORF2 gene sequence. All norovirus strains detected were classified into seven genotypes, six of which belonged to genogroup GII. GII.2 was the dominant genotype till February 2017, whereas GII.4 prevailed thereafter. Most of the GII.4 strains were of the Sydney_2012 variant, whereas five strains could not be classified. Further recombination analysis at the ORF1/ORF2 gene junction revealed that 23 out of 24 strains were recombinant, thereby showcasing the significant role of genetic recombination in norovirus evolution and epidemiology. Continuous genomic surveillance and molecular characterization are essential for tracking norovirus evolution, which could contribute to the elucidation of new aspects of virus-host interactions that potentially affect host morbidity and epidemiology.

Biological and physico-chemical characterization of human norovirus-like particles under various environmental conditions

Human noroviruses (HuNoVs) are the predominant etiological agent of viral gastroenteritis in all age groups worldwide. Mutations over the years have affected noroviruses' responses to environmental conditions due to the arrangement of amino acid residues exposed on the VP1 capsid surface of each strain. The GII.4 HuNoV genotype has been the predominant variant for decades, while the GII.17 genotype has often been detected in East Asia since 2014. Here, GII.17 and GII.4 baculovirus-expressed VLPs (virus-like particles) were used to study the biological (binding to HuNoV ligand, namely the ABO and Lewis antigens) and physicochemical properties (size, morphology, and charge) of the HuNoV capsid under different conditions (temperature, pH, and ionic strength). GII.17 showed stability at low and high ionic strength, while GII.4 aggregated at an ionic strength of 10 mM. The nature of the buffers influences the morphology and stability of the VLPs. Here, both VLPs were highly stable from pH 7-8.5 at 25 °C. VLPs retained HBGA binding capability for the pH, ionic strength and temperature encountered in the stomach (fed state) and the small intestine. Increasing the temperature to above 65 °C altered the morphology of VLPs, causing aggregation, and decreased their affinity to HBGAs. Comparing both isolates, GII.17 showed a better stability profile and higher affinity to HBGAs than GII.4, making them interesting candidate particles for a future norovirus vaccine. Biological and physicochemical studies of VLPs are as pertinent as ever in view of the future arrival of VLP-based HuNoV vaccines.

Use of Zebrafish Embryos To Reproduce Human Norovirus and To Evaluate Human Norovirus Infectivity Decay after UV Treatment

This study reports an essential improvement of the method for replication of human norovirus (HNoV) with the use of zebrafish (Danio rerio) embryos. With three HNoV genotypes and P-types GII.2[P16], GII.4[P16], and GII.17[P31], we demonstrated that this tool had higher efficiency and robustness than the zebrafish larvae as reported previously. When zebrafish larvae were injected with virus (1.6 ± 0.3 log genome copies/10 larvae), a significant increase of virus genome copies was detected at 2 days postinfection (dpi; 4.4 ± 0.8 log genome copies/10 larvae, P < 0.05) and the viral loads started to decrease gradually from 3 dpi. In comparison, when the viruses were injected into the zebrafish embryos, significant virus replication was noticed from 1 dpi and lasted to 6 dpi (P < 0.05). The virus levels detected at 3 dpi had the highest mean value and the smallest variation (7.7 ± 0.2 log genome copies/10 larvae). The high levels of virus replication enabled continuous passaging for all three strains up to four passages. The zebrafish embryo-generated HNoVs showed clear patterns of binding to human histo-blood group antigens (HBGAs) in human saliva by a simple saliva-binding reverse transcription-quantitative PCR (RT-qPCR). Last, in a disinfection study, it was shown that a dose of 6 mJ/cm2 UV254 was able induce a >2-log reduction in HNoV infectivity for all three HNoV strains tested, suggesting that HNoVs were more UV susceptible than multiple enteric viruses and commonly used HNoV surrogates as tested before. IMPORTANCE HNoVs are a leading cause of gastroenteritis outbreaks worldwide. The zebrafish embryo tool as developed in this study serves as an efficient way to generate viruses with high titers and clean background and a straightforward platform to evaluate HNoV inactivation efficacies. It is expected that this tool will not only benefit epidemiological research on HNoV but also be used to generate HNoV inactivation parameters which are highly needed by the water treatment and food industries.

Epochal coevolution of minor capsid protein in norovirus GII.4 variants with major capsid protein based on their interactions over the last five decades

Norovirus is a leading cause of viral gastroenteritis outbreaks worldwide, with GII.4 responsible for the majority of infections. Minor capsid protein VP2 has been found to have functions such as stabilizing virus particles, and VP2 is one of the highly variable proteins of norovirus, similar to major capsid protein VP1. However, whether the variation of VP2 is functionally driven still remains unclear. In this study, VP2 showed a higher evolutionary rate (2.642×10^-3 substitutions/site/year) than VP1 (1.587×10^-3 substitutions/site/year), and a hypervariable region in VP2 in a serial of norovirus GII.4 over the past 50 years had been observed. Notably, the high variation of VP2 was not haphazard. The evolutionary process of VP2 is similar to that of VP1 with comparable topologies when the phylogenetic trees were constructed. Moreover, VP2 was found to interact with VP1 among epidemic variants of GII.4 using the yeast two-hybrid experiments. The results of interactions were grouped into time-adjacent (e.g. Ancestral-VP1 plus US95-VP2) and non-adjacent (e.g. Ancestral-VP1 plus Sydney-VP2) according to the epochal chronologically based prevalence of GII.4 norovirus. Interestingly, the interaction of the former group was significantly stronger than that of the latter group (P=0.0001). Furthermore, the interaction regions on VP2 (residues 131-160 and 171-180) were mapped to the hypervariable region. And these interaction regions did show an important role in the evolutionary process of VP2, which was consistent with that of VP1. In summary, the minor capsid protein VP2 of GII.4 noroviruses had shown the epochal coevolution with VP1 based on their interactions over the past 50 years. The findings of this study provided valuable information for further understanding and completing the evolutionary mechanism of norovirus.